The present invention relates to a defect inspection method of inspecting a minute pattern formed on a substrate by a thin-film forming process, such as a semiconductor device fabricating process or a flat panel display manufacturing process, for defects formed in the pattern and contaminants adhering to the pattern, and a defect inspection apparatus for carrying out the same.
A semiconductor device inspecting apparatus is disclosed in Int. Pat. Pub. No. WO99/06823. This known semiconductor device inspecting apparatus focuses a radiation beam fallen on a surface of a wafer at an incident angle in a line on the surface of the wafer. Light scattered by an illuminated linear area is received by an optical detector, and an image sensor forms a scattered image. A central part of the image sensor is a flat surface substantially perpendicular to the surface on which the flat illuminating beam falls. Thus, scattered light received by the image sensor is lateral scattered light. The image formed by the image sensor is compared with an image of an adjacent die on which the same pattern is formed, to determine whether or not the pattern is defective.
Wafers subjected to inspection are provided with various patterns, respectively, and various kinds of defects are formed by various causes. The size of defects has been progressively diminished with the progressive miniaturization of patterns formed by semiconductor device fabricating processes. Generally, the quantity of scattered light is proportional to the sixth power of particle size. Therefore, the quantity of scattered light from defects decreases as the defects are miniaturized. When a dark field detection system is used, an image of a pattern is brighter than those of defects if a scattering cross section of the pattern is greater than defects. Therefore, the intensity of illuminating light is adjusted so that the brightness of an image of the pattern may not saturate the image sensor. In such a case, detection of defects through image processing is difficult because the brightness of images of defects in the image of the pattern is low. If the intensity of illuminating light is increased such that the brightness of the image of the pattern is higher than the saturation level of the image sensor to form images of defects in high brightness, it is difficult to detect the defects on the pattern because brightness of the image of the pattern is higher than the saturation level of the image sensor.
An image suitable for high-sensitivity inspection can be obtained by forming the image of the pattern in a moderate brightness to enhance the brightness of the images of the defects relatively. The technique mentioned in Int. Pat. Pub. No. WO99/06823 focuses a flat beam in a bright line on the surface of a wafer to illuminate a linear area in the surface of the wafer. In this state, reflected light from a part extending in a direction perpendicular to the length of the illuminating line of the pattern is distributed in a width corresponding to the NA (numerical aperture) of focused illumination on a Fourier transform plane formed by an objective lens. When the reflected light from the pattern is filtered by a spatial filter, the aperture of the objective lens is limited, and hence, the resolution of the objective is deteriorated.
The technique mentioned in Int. Pat. Pub. No. WO99/06823 uses an oblique detecting system having an optical axis inclined at an angle to a normal direction to the surface of the wafer. When the angle between the normal direction to the surface of the wafer and the optical axis of the oblique detecting system and the NA of the illuminating light are in some relationship, an optical system can be disposed so that the reflected light from a part extending in a direction perpendicular to the length of the illuminating line of the pattern is not detected. When the NA of the oblique detecting system is increased to improve the resolution, i.e., to increase the intensity of scattered light from small defects, the NA cannot be increased because the wafer interferes mechanically with the objective.